Catalytic Nitrous Oxide Reduction with H2 Mediated by Pincer Ir Complexes

Reduction of nitrous oxide (N2O) with H2 to N2 and water is an attractive process for the decomposition of this greenhouse gas to environmentally benign species. Herein, a series of iridium complexes based on proton-responsive pincer ligands (1–4) are shown to catalyze the hydrogenation of N2O under mild conditions (2 bar H2/N2O (1:1), 30 °C). Among the tested catalysts, the Ir complex 4, based on a lutidine-derived CNP pincer ligand having nonequivalent phosphine and N-heterocyclic carbene (NHC) side donors, gave rise to the highest catalytic activity (turnover frequency (TOF) = 11.9 h–1 at 30 °C, and 16.4 h–1 at 55 °C). Insights into the reaction mechanism with 4 have been obtained through NMR spectroscopy. Thus, reaction of 4 with N2O in tetrahydrofuran-d8 (THF-d8) initially produces deprotonated (at the NHC arm) species 5NHC, which readily reacts with H2 to regenerate the trihydride complex 4. However, prolonged exposure of 4 to N2O for 6 h yields the dinitrogen Ir(I) complex 7P, having a deprotonated (at the P-arm) pincer ligand. Complex 7P is a poor catalytic precursor in the N2O hydrogenation, pointing out to the formation of 7P as a catalyst deactivation pathway. Moreover, when the reaction of 4 with N2O is carried out in wet THF-d8, formation of a new species, which has been assigned to the hydroxo species 8, is observed. Finally, taking into account the experimental results, density functional theory (DFT) calculations were performed to get information on the catalytic cycle steps. Calculations are in agreement with 4 as the TOF-determining intermediate (TDI) and the transfer of an apical hydrido ligand to the terminal nitrogen atom of N2O as the TOF-determining transition state (TDTS), with very similar reaction rates for the mechanisms involving either the NHC– or the P–CH2 pincer methylene linkers.


■ INTRODUCTION
While representing only the 6% of gases causing climate warming released to earth's atmosphere, nitrous oxide (N 2 O) is a potent greenhouse gas with an estimated warming impact 300 times that of carbon dioxide (CO 2 ). 1 Moreover, N 2 O has a relevant influence on the depletion of the stratospheric ozone. 2 Anthropogenic N 2 O, mainly produced from the use of nitrogen-containing fertilizers, biomass and fossil fuel combustion, and industrial chemical processes, such as adipic and nitric acid syntheses, has been claimed as the main source for the upward trend in its atmospheric concentration in the last decades. 3 Therefore, strategies developed to mitigate its concentration, either by lowering its emissions or by implementing processes that can degrade it to chemical species with low environmental impact, are drawing increased attention. 4 Decomposition of N 2 O to N 2 and O 2 is a thermodynamically favorable process, albeit challenging due to their associated high kinetic barriers. 5 Alternatively, nitrous oxide hydrogenation represents an appealing approach to the degradation of N 2 O to N 2 and water due to the future prospects of the development of clean, large-scale processes for H 2 production from renewable sources. 6 Metal-based heterogeneous catalysts have been shown to catalyze the reduction of N 2 O with H 2 , usually under relatively harsh conditions. 7 Conversely, although activation of N 2 O by transition-metal complexes 8 and subsequent reduction with H 2 to innocuous N 2 and water have been investigated, 9−13 there is a lack of competent homogeneous catalytic systems for this reaction (Figure 1). Seminal work by Bergman et al. demonstrated the feasibility of a stepwise hydrogenation of N 2 O. The reaction of Ru(DMPE) 2 H 2 (DMPE = 1,2-bis(dimethylphosphino)ethane) with 1 equiv of N 2 O afforded the hydroxo complex Ru(DMPE) 2 H(OH), which upon reaction with H 2 regenerated the initial Ru dihydride derivative. 9 Later, Caulton et al.
carried out the reaction of Os(PNP)H 3 (PNP = N-(SiMe 2 CH 2 PtBu) 2 ) with N 2 O to yield Os(PNP)H(N 2 ) and H 2 O. 10 Reaction of the later complex with H 2 led to the slow formation of the initial Os(PNP)H 3 derivative with the expected release of N 2 . However, complete catalytic studies with this system were not performed. More recently, Piers et al. reported the reaction of an iridium pincer carbene complex, Ir(PC sp2 P)Cl, with N 2 O to afford an iridaepoxide species resulting from the addition of the N 2 O oxygen atom to the Ir�C bond. 11 This derivative was shown to react with H 2 to release H 2 O upon heating. Also, interestingly, Gruẗzmacher et al. developed a dehydrogenative coupling of alcohols using N 2 O as a hydrogen scavenger catalyzed by a Rh complex featuring a proton-responsive bis(olefin)amido ligand. 12 Formation of N 2 was observed in the reaction of N 2 O with H 2 in the presence of the Rh complex.
Only in 2017, Milstein et al. reported the use of several proton-responsive Ru complexes in the catalytic hydrogenation of N 2 O. 13 Among the tested catalysts, a lutidine-derived Ru− PNP complex led to the highest catalytic activity, providing a turnover number (TON) value of 417 in 48 h at 65°C (P = 3 bar N 2 O + 4 bar H 2 ) ( Figure 1a). In the absence of H 2 , catalyst degradation to a complex mixture was observed under N 2 O atmosphere. More recently, Trincado, Gianetti, Gruẗzmacher et al. developed a bimetallic Pt−Rh complex containing a multidentate phosphine/olefin/bis-alkyne ligand that was found to catalyze the hydrogenation of nitrogen oxides (NO 2 , NO, and N 2 O) ( Figure 1b). 14 Particularly, reduction of N 2 O was accomplished at room temperature under low pressure (P = 2 bar N 2 O + 2 bar H 2 ) with TONs of up to 587 in 48 h (TOF = 12.2 h −1 ).
Considering these precedents, and the fact that diverse Ir complexes have been shown to catalyze the hydrogenation of polar organic compounds and CO 2 , 15−18 we examined the catalytic behavior of a series of Ir complexes (1−4) incorporating proton-responsive pincer ligands in the hydrogenation of N 2 O ( Figure 2). Herein, in addition to the comparison of the catalytic activities of these complexes, experimental and theoretical mechanistic studies of the reaction catalyzed by the most active catalyst, a lutidinederived Ir−CNP complex (Figure 1c), are reported. ■ RESULTS AND DISCUSSION Catalyst Screening. Initially, the polyhydride iridium complexes containing proton-responsive ligands 1−4 were tested in the hydrogenation of N 2 O ( Figure 2). Reactions were carried out at 30°C in tetrahydrofuran (THF) using 2 bar of a 1:1 H 2 /N 2 O mixture for 20 h (Table 1). Under these conditions, the reaction catalyzed by the Ir complex 1, based on a diethylamino-derived PN H P ligand, 15 proceeded with a TON of 78.4 (entry 1). Using the lutidine-derived Ir−PNP complex 2, 16 15.3 turnovers were achieved under the examined   reaction conditions (entry 2). Meanwhile, the κ 4 -(P,N Py ,C NHC ,C aryl ) lutidine-derived iridium complex 3 17 gave rise to a similar low conversion (entry 3). Finally, the iridium complex 4, incorporating a nonsymmetric lutidine-derived CNP pincer ligand containing a phosphino group and an Nheterocyclic carbene (NHC) as side arms, 18 displayed the highest catalytic activity within the catalysts examined (TON = 238; TOF = 11.9 h −1 ) (entry 4). With 4, a slight improvement in the catalytic activity was observed upon increasing the reaction temperature to 55°C, reaching a TON of 328 (entry 5); while carrying out the reaction for 4 days at 30°C, up to 525 catalyst turnovers were observed with a decrease of the catalytic activity (TOF = 5.5 h −1 ) (entry 6). The reduced activity observed with complex 4 upon longer reaction times suggests that catalyst deactivation takes place. A catalyst deactivation pathway has been elucidated through the realization of mechanistic studies (vide infra). Finally, when the reaction was performed in the presence of benzene, arene hydrogenation products were not observed, ruling out the formation of nanoparticles (entry 7), whereas the Hg poisoning test also supported the homogeneous nature of the catalytic process (entry 8). 19 Stoichiometric Reactions. Previously, we have reported on the ability of the P-and NHC-CH 2 linkers of 4 to get involved in reversible ligand-assisted H 2 elimination/activation. 18 Scrambling experiments using D 2 /H 2 produced the reversible deuteration of the methylene arms of the pincer and the Ir−H hydrogens, pointing out to the existence of a reversible exchange of free D 2 with a η 2 -H 2 ligand produced from the intramolecular protonation of the hydrido ligands by the CH 2 −NHC and CH 2 −P methylene linkers (Scheme 1). 20 To get further information on the reactivity of 4 toward the catalytic reaction partners, a solution of the complex in THF-d 8 was pressurized with N 2 O (1.5 bar) and analyzed using 1 H and 31 P{ 1 H} NMR spectroscopies (Scheme 2). After approximately 30 min, formation of a new major species, 5 NHC , was observed that produces in the 1 H NMR spectrum a broad resonance at −23.7 ppm and a singlet at δ P 74.3 ppm in the 31 P{ 1 H} NMR experiment. The dihydride complex 5 NHC is deprotonated at the pincer ligand NHC arm, and its formation likely involves a ligand-assisted transfer of dihydrogen to N 2 O. Confirmation of the proposed structure of 5 NHC was achieved through the reaction of the chlorodihydride derivative 6 with KHMDS (1.3 equiv) in THF-d 8 (Scheme 3). 21 In addition to a doublet at −23.77 ppm ( 2 J HP = 11.3 Hz) due to the IrH hydrogens, the 1 H NMR spectrum of 5 NHC features a singlet resonance for the proton of the methine CHN bridge at 6.44 ppm and a doublet at 2.98 ppm ( 2 J HP = 8.9 Hz) caused by the CH 2 P arm. Moreover, pyridine ring dearomatization is reflected in the significant upfield shift of the central N-heterocycle hydrogens appearing in the range 6.2−5.7 ppm. Spin-lattice relaxation time (T 1 ) 22 determinations in THF-d 8 shows a minimum T 1,min value of approximately 150 ms, in agreement with a "classical" dihydride formulation of complex 5 NHC . 23 Finally, it is interesting to note that exposure of a THF-d 8 solution of 5 NHC to H 2 (1.5 bar) produced the instantaneous regeneration of the trihydride complex 4 (Scheme 2).
Prolonged exposure to N 2 O (1.5 bar) of a THF-d 8 solution of complex 4 for 6 h produced the formation of a major species appearing in the 31 P{ 1 H} NMR spectrum as a singlet at δ P 57.3 ppm. This derivative was characterized as the terminal dinitrogen Ir(I) complex 7 P (Scheme 2). Selective deprotonation of the pincer ligand at the P-bound bridge was evidenced by the presence in the 1 H NMR spectrum of a singlet signal at 4.58 ppm (integrating to 2H) corresponding to the CH 2 − NHC linker, and a doublet resonance at 3.47 ppm ( 2 J HP = 2.3 Hz, 1H) produced by the methine CHP arm. As with 5 NHC , the resonances corresponding to the pyridine-derived fragment appear significantly shifted upfield (6.05−5.13 ppm) in the 1 H NMR spectrum, suggestive of ring dearomatization. The presence of the coordinated N 2 ligand is deduced by a strong absorption at 2081 cm −1 in the IR spectrum attributed to the N−N stretching of the terminally bound dinitrogen. 24 Density functional theory (DFT) calculations (B3LYP-D3, 6-31g(d,p)/ SDD) of 7 P indicated that this species is more stable than its Scheme 1. Reversible Deuteration of 4 with D 2 Scheme 2. Formation of Complexes 5 NHC and 7 P , and the Reaction of 5 NHC with H 2 Scheme 3. Formation of Complex 5 NHC by the Reaction of 6 with KHMDS Inorganic Chemistry pubs.acs.org/IC Article tautomer deprotonated at the NHC linker, 7 NHC , by 4.1 kcal/ mol. X-ray diffraction analysis of a crystal of 7 P revealed a squareplanar coordination geometry (∑(Ir) = 361.6°), with quite similar metric parameters to the Ir-pincer framework of the previously reported Ir(CNP Ph *)(CO) complex ( Figure 3). 25 The Ir−N−N angle of 174.4°is in agreement with the proposed end-on coordination of the N 2 ligand. Deprotonation of the methylene P-linker is evidenced by the relatively short C(19)−P(1) and C(19)−C(18) distances of 1.759 and 1.365 Å, respectively. Moreover, the pyridine moiety exhibits alternating C−C bond lengths in agreement with substantial ring dearomatization, as reflected by the elongated C(18)− C(17) and C(16)−C(15) distances of 1.458 and 1.445 Å, and shortened C(17)−C(16) and C(15)−C(14) bond lengths of 1.339 and 1.371 Å, respectively (average C−C bond in pyridine: 1.38 Å). Finally, the N−N bond length of 1.108 Å of the N 2 ligand suggests a modest dinitrogen activation (N−N distance in free N 2 : 1.098 Å). 24 To determine whether 7 P could be an intermediate in the N 2 O hydrogenation catalyzed by 4, the complex was tested under the standard conditions employed for the comparison of catalysts 1−4. A much lower catalytic activity (TON = 22.8) than that provided by 4 was observed (Table 1, entry 9), suggesting that formation of 7 P is a potential catalyst deactivation route. It should be noted, however, that reaction of the deprotonated species 5 NHC with H 2 is much faster than with N 2 O, and consequently, catalyst deactivation is only observed upon prolonged exposure to N 2 O.
Experimental and theoretical investigations of the hydrogenation of N 2 O with lutidine-derived PNP−Ru catalysts have shown the formation of hydroxo derivatives as reaction intermediates. 13,26,27 Attempts to independently synthesize an Ir analogue by the addition of water to solutions of 5 NHC , formed in situ from the reaction of 6 with base, were unsuccessful. However, when the reaction of 4 with N 2 O was carried out in wet THF-d 8 , instead of the expected signal for 5 NHC in the hydride region of the 1 H NMR spectrum, two mutually coupled doublets of doublets appearing at −20.34 ppm ( 2 J HP = 14.5 Hz, 2 J HH = 6.3 Hz) and −24.02 ppm ( 2 J HP = 17.3 Hz, 2 J HH = 6.7 Hz) were observed (Scheme 4). These resonances were assigned to the hydroxo species 8 on the basis of the similar pattern and chemical shifts of its hydride resonances and 31 P NMR spectrum chemical shift (δ P 65.0 ppm) to those of the chlorodihydride derivative 6. In addition, resonances attributable to the Ir(I) complex 7 P were also observed in both the 1 H and 31 P{ 1 H} NMR spectra (approx. ratio 7 P /8 = 2.5). Subsequent pressurization of the solution with H 2 (1.5 bar), without previous removal of the N 2 O atmosphere, produced the instantaneous complete transformation of 8 to the trihydride complex 4, with 7 P remaining unchanged.
DFT Calculations. Taking into account the above experimental results, a mechanism for the hydrogenation of N 2 O catalyzed by 4 involving an outer-sphere, ligand-assisted hydrogen transfer can be assumed. The likeness of such a mechanism was further investigated theoretically by the performance of DFT calculations (B3LYP-D3, 6-31g(d,p)/ SDD). Since reaction pathways involving both the CH 2 −P and CH 2 −N linkers of the pincer are feasible, 18,28 independent mechanisms with the two ligand arms were considered ( Figure  4). Thus, in line with previous reports on the insertion of N 2 O into Ru−H bonds, 26,27 transfer of each of the two apical hydrido ligands to the terminal nitrogen atom of N 2 O was examined, leading to the endergonic formation of the cationic species A P and A NHC , respectively. The transition states TS 4→A(P) and TS 4→A(NHC) associated with these processes have very similar energies (ΔG ‡ ) of ca. 18 kcal/mol ( Figure 5). Subsequent coordination of HN�N�O − through the oxygen atom to the Ir center of A P and A NHC takes place with modest energy barriers of 2.9 and 0.7 kcal/mol (TS A(P)→B(P) and TS A(NHC)→B(NHC) , respectively), leading to the neutral complexes B P and B NHC . The overall energy returns for the formation of these species from 4 are 5.7 (B P ) and 9.1 (B NHC ) kcal/mol.   Figure  6). The formation of 8 P and 8 NHC from B P and B NHC , respectively, is highly exergonic by ca. 51 kcal/mol. Subsequent intramolecular protonation of the hydroxo species 8 by the CH 2 −P or CH 2 −N arms produces the Inorganic Chemistry pubs.acs.org/IC Article formation of the aquo complexes 5 P ·H 2 O and 5 NHC ·H 2 O through relatively low barriers (10.1 and 9.9 kcal/mol), which after H 2 O decoordination lead to the deprotonated species 5 P and 5 NHC , respectively. As previously calculated, 18 species 5 P and 5 NHC are able to activate H 2 in a ligand-assisted process, after initial formation of the dihydrogen complexes 5 P ·H 2 and 5 NHC ·H 2 , to yield the trihydride complex 4 and close the catalytic cycle. This last step has a lower energy barrier for 5 NHC (7.6 kcal/mol) than for 5 P (ΔΔG ‡ = 5.8 kcal/mol). However, since the participation of water molecules has been shown to decrease the barrier of the intramolecular H 2 activation in related lutidine-derived Ir complexes, 29 the formation of the trihydride complex 4 from 5 P ·H 2 and 5 NHC · H 2 was also examined in the presence of explicit H 2 O molecules (Figure 7). The successive addition of one to three water molecules accelerates the cleavage of dihydrogen, lowering the barrier of the process from 5 P ·H 2 and 5 NHC ·H 2 to only 2.7 kcal/mol ( Figure 8). Analysis of the DFT calculations profiles using the energetic span model proposed by Kozuch and Shaik 30 provides energetic spans (δE) for the calculated mechanisms involving the NHC− and P−CH 2 methylene linkers of 18.0 and 18.5 kcal/mol, respectively, being 4 the TOF-determining intermediate (TDI) and TS 4→A(NHC) and TS 4→A(P) the TOFdetermining transition states (TDTS). These results show very similar reaction rates for the mechanisms involving both pincer methylene linkers. In addition, comparison of the two relevant steps of the N 2 O reduction stage (TS 4→A and TS B→8 ) for the Ir−CNP system with those calculated for the Milstein's Ru− PNP complex shows slightly lower energy barriers for the former catalyst. To summarize, the lutidine-derived CNP iridium complex 4, having two nonequivalent Brønsted acid/base sites, catalyzes the hydrogenation of N 2 O with relatively high TOF values of up to 11.9 h −1 at 30°C, and 16.4 h −1 at 55°C. These catalytic activities are higher than those provided by other iridium complexes incorporating proton-responsive pincer ligands. More interestingly, the catalytic activity provided by 4 is comparable to that of the two only homogeneous catalytic systems reported for N 2 O hydrogenation, a lutidine-derived PNP−Ru catalyst reported by Milstein et al. and the PtRh bimetallic complex described by Trincado, Gianetti, Gruẗzmacher et al. Experimental and theoretical investigations of the reaction mechanism indicate that both CH 2 −NHC and CH 2 − P methylene bridges of the pincer can be involved in the key ligand-assisted processes of the catalytic reaction. Finally, a catalyst deactivation pathway involving the formation of the dinitrogen Ir(I) complex 7 P from the reaction of the catalyst precursor 4 with N 2 O has been identified. ■ EXPERIMENTAL SECTION General Procedures. All reactions and manipulations were performed under nitrogen or argon, either in a Braun Labmaster 100 glovebox or using standard Schlenk-type techniques. All solvents were distilled under nitrogen with the following desiccants: sodiumbenzophenone-ketyl for tetrahydrofuran (THF and THF-d 8 ), and  Inorganic Chemistry pubs.acs.org/IC Article sodium for pentane and benzene-d 6 . Iridium complexes 1, 15 2, 16 3, 17 and 4 18 were prepared as previously described. All other reagents were purchased from commercial suppliers and used as received. NMR spectra were obtained on DRX-400 and AVANCEIII/ASCEND 400R spectrometers. 31 P{ 1 H} NMR shifts were referenced to external 85% H 3 PO 4 , while 13 C{ 1 H} and 1 H shifts were referenced to the residual signals of deuterated solvents. All data are reported in ppm downfield from Me 4 Si. All NMR measurements were carried out at 25°C, unless otherwise stated. NMR signal assignations were confirmed by two-dimensional (2D) NMR spectroscopy ( 1 H− 1 H correlated spectroscopy (COSY), 1 H− 1 H nuclear Overhauser effect spectroscopy (NOESY), 1 H− 13 C heteronuclear single quantum coherence (HSQC), and 1 H− 13 C heteronuclear multiple bond correlation (HMBC)) for all of the complexes. IR spectra were acquired on a Bruker Tensor 27 instrument. GC−MS analysis was carried out using a Shimadzu GCMS-TQ8040 apparatus equipped with a PoraBOND Q capillary column (25 m, 0.25 mm i.d., 3 μm film thickness). Helium carrier gas was supplied at a head pressure of 3.7 psi to provide an  Inorganic Chemistry pubs.acs.org/IC Article initial flow rate of 4.6 mL/min. The injector temperature was set up to 200°C, and the oven temperature was initially held at 30°C for 5 min, then gradually increased to 150°C at 25°C/min. Full-scan mass spectra were collected from m/z 10 to 50 at a data acquisition rate of 158 spectra/s. The MS transfer line was held at 250°C, and the ion source temperature was 200°C. Computational Details. Calculations were carried out at the DFT level using the Gaussian 09 program 31 with the B3LYP hybrid functional, 32 with dispersion effects taken into account by adding the D3 version of Grimme's empirical dispersion. 33 All atoms were represented with the 6-31g (d,p) basis set, 34 except Ir, for which the Stuttgart/Dresden Effective Core Potential and its associated basis set SDD 35 was used. All geometry optimizations were performed in bulk solvent (THF) without restrictions. Vibrational analysis was used to characterize the stationary points in the potential energy surface, as well as for calculating the zero-point, enthalpy, and Gibbs energy corrections at 295 K and 1 atm. The nature of the intermediates connected by a given transition state along a reaction path was proven by intrinsic reaction coordinate (IRC) calculations or by perturbing the geometry of the TS along the reaction path eigenvector. Bulk solvent effects were modeled with the SMD continuum model. 36 Representative Procedure for the Hydrogenation of N 2 O. In a glovebox, a Fisher−Porter vessel (25 mL) was charged with a solution of complex 4 (1.0 mg, 1.6 μmol) and mesitylene (5.0 μL, 35.9 μmol) in THF (0.6 mL). The nitrogen atmosphere in the reactor was replaced by 1 bar of H 2 by performing three freeze−pump−thaw cycles, and the vessel was further pressurized with N 2 O until a total gauge pressure of 2 bar was achieved (N 2 O/H 2 ratio = 1:1) and heated to 30°C. After 20 h, the gas atmosphere was analyzed by GC− MS to detect N 2 formation. The reactor was depressurized, and the solution was transferred under inert atmosphere to an NMR tube containing a coaxial insert filled with C 6 D 6 . Conversion was determined through 1 H NMR spectroscopy by integrating the H 2 O signal using mesitylene as internal standard.
Complex 7 P . NMR Scale. In a J. Young valved NMR tube, a solution of 4 (0.012 g, 0.02 mmol) in THF-d 8 (0.5 mL) was pressurized with N 2 O (2.5 bar). The sample was analyzed by NMR spectroscopy after 6 h, allowing us to observe the formation of complex 7 P (Figure 10). Crystals of complex 7 P suitable for X-ray diffraction were obtained after evaporation of the solvent, extraction of the residue with pentane, and cooling off the resulting solution to −20°C.
Preparative Scale. A Fisher−Porter vessel (25 mL) was charged with a solution of complex 4 (0.070 g, 0.11 mmol) in THF (2.5 mL). The nitrogen atmosphere in the reactor was replaced by 2.5 bar of N 2 O by performing three freeze−pump−thaw cycles, and the solution was stirred overnight. The reactor was depressurized, and the solution was transferred to a Schlenk flask and brought to dryness under reduced pressure. The residue was extracted with pentane (2 × 5 mL), and the resulting solution was cooled to −20°C. The precipitated solid was filtered and washed with cold pentane (3 × 2 mL) to yield a yellow solid (0.040 g; 56%). Attempts to obtain analytically pure samples of 7 P were unsuccessful.  (d, 3 J HP = 12.7 Hz, 18H, 2 C(CH 3 ) 3 ). 31  The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.2c02963.
Selected NMR spectra, experimental procedures, DFT calculations details, and X-ray diffraction data (PDF) Cartesian coordinates, absolute Gibbs energies (THF), and imaginary frequencies (TSs) of the DFT-optimized structures (collective XYZ file) (XYZ)